I think the Rowdyruff Boys have a distinct flavor that's different from the Powerpunk Girls. The Rowdyruffs are still just as childish as the Powerpuffs, but in a mischievous schoolboy sort of way. Putting a spider in a girl's hair, roughhousing, trying to look tough, etc. The Rowdyruffs can even be good sometimes. The Powerpunk Girls are like the true negative of the Powerpuff Girls: They're still girly, but they're bitchy, nasty and magnify the specific worst traits of each of their counterparts. Blossom as a bossy manipulative snob, Bubbles as a spoiled selfish crybaby and Buttercup as a total sadist.
Comparing the Powerpunks to the Rowdyruffs is like the comparison between Justice Lords Superman and Bizarro Superman, or Negaduck and Darkwarrior Duck. Yes they share the concept of a bad guy version of said character, but each approaches the idea in a vastly different way.